Does the order in Scala for loop influence

Question

I'm new for Scala. I have a question about the order in the foor loop.

type Occurrences = List[(Char, Int)]

lazy val dictionaryByOccurrences: Map[Occurrences, List[Word]] = dictionary.groupBy(x => wordOccurrences(x))

def wordAnagrams(word: Word): List[Word] = dictionaryByOccurrences.getOrElse(wordOccurrences(word), List())

def combinations(occurrences: Occurrences): List[Occurrences] = occurrences match {
    case List() => List(List())
    case head::tail => {
    for (o <- combinations(tail); x <- 1 to head._2)
    yield (head._1, x) :: o
}

if I change the order in the for loop, it will be wrong

def combinations(occurrences: Occurrences): List[Occurrences] = occurrences match {
    case List() => List(List())
    case head::tail => {
    for (x <- 1 to head._2; o <- combinations(tail))
    yield (head._1, x) :: o
}

I can not find the reason

score 1 · Answer 1 · edited May 23 '17 at 12:31

The type constructor of for(x <- xs; y <- ys; ...) yield f(x, y, ...) is the same of xs by default. Now the return type of combinations is List[Occurrences], then the expected type constructor is List[_], while the type constructor of 1 to n is Seq[_].

The following codes work:

def combinations(occurrences: Occurrences): List[Occurrences] = occurrences match {
    case List() => List(List())
    case head::tail => {
    for (x <- (1 to head._2).toList; o <- combinations(tail))
    yield (head._1, x) :: o
}

And this would work too:

import collection.breakOut
def combinations(occurrences: Occurrences): List[Occurrences] = occurrences match {
    case List() => List(List())
    case head::tail => {
      (for (x <- 1 to head._2; o <- combinations(tail))
        yield (head._1, x) :: o)(breakOut)
    }
}

In depth, for(x <- xs; y <- ys; ...) yield f(...) is equivalent to xs.flatMap(...), and the full signature of List#flatMap as following:

def flatMap[B, That](f: (A) ⇒ GenTraversableOnce[B])
                    (implicit bf: CanBuildFrom[List[A], B, That]): That

You can see that the return type of flatMap is a magic That which is List[B] by default, you can look into Scala 2.8 CanBuildFrom for more information.

Does the order in Scala for loop influence

1 Answers1